The present invention is related to substrate processing equipment and more particularly to plasma processing equipment for performing plasma processing steps such as deposition, cleaning, and/or etch processes on a process substrate.
It is well known that plasma discharges may be used to excite gases to produce activated gases containing ions, free radicals, atoms, and molecules. Such activated gases are used for numerous industrial applications, including, in particular, various operations performed during the fabrication of semiconductor devices. For example, plasma-processing methods are used in deposition processes, such as plasma-enhanced chemical vapor deposition (PECVD) or high-density-plasma chemical vapor deposition (HDP-CVD), to deposit layers of material on substrates. Plasma-processing methods are also used within a number of etching techniques, such as reactive ion etching (RIE) or deep RIE (DRIE). Plasmas are also used in cleaning processes to prepare a processing chamber or the surface of a particular substrate for subsequent processes; such processes include a plasma wafer surface clean or activation prior to formation of a layer on the surface.
Generally, plasma-processing applications can be characterized by the kinetic energy of the ions in the plasma and by the level of direct exposure the material being processed has to the plasma. For example, applications sensitive to material damage generally require low-kinetic-energy ions and/or shielding of the material from the plasma, while applications such as anisotropic etching require ions with high kinetic energy. Certain applications, such as RIE or DRIE require relatively precise control of the ion energy. Applications such as generating ion-activated chemical reactions, and etching or deposition of material into high-aspect-ratio structures, are examples of processes that make use of direct exposure of the material to a high-density plasma.
This wide application of plasma processing uses is reflected in the extensive variety of available plasma processing systems and apparatuses. The basic methods these systems use for plasma generation include dc discharge, RF discharge, and microwave discharge. One particular type of plasma processing chamber places the wafer on an electrode of the plasma circuit, opposite another planar electrode, and capacitively couples high-frequency electrical power to the two electrodes to form a plasma between them. Such a plasma reactor has advantages where it is desirable to form the plasma in the presence of the substrate, such as when the physical movement of plasma species to and from the substrate is specifically desired. However, some devices or materials are not readily compatible with this type of plasma formation, particularly because the plasma includes high-energy photons and their direct bombardment on the substrate results in undesirable heating. Another approach to plasma processing generates plasma in a remote location and couples the plasma to a processing chamber. Various types of remote plasma generators have been developed, including magnetron sources coupled to a cavity, inductively coupled toroidal sources, microwave irradiation directed at a plasma precursor, electron-cyclotron resonance generators, and others. For particular types of processes, such as cleaning processes, remote plasma techniques offer certain advantages.
Inductively coupled RF plasma systems are often used in processing semiconductor wafers, in part because they can generate large-area plasmas. In principle, inductively coupled plasma systems permit generation of a high-density plasma in one portion of a processing chamber (e.g. above the material being processed) and simultaneous shielding of the material from the plasma-generation region. Such systems attempt to use the plasma itself as a protective buffer that protects the material from various possible deleterious plasma effects attributable to characteristics of the plasma-generation region. Because the drive currents are only weakly coupled to the plasma, however, these plasmas cannot be made absolutely inductive and require high voltages on drive coils to compensate for the resulting inefficiency. These high voltages produce large electrostatic fields that cause high-energy ion bombardment, primarily on the reactor surfaces, but also on the material being processed.
Approaches to shield the electrostatic fields have included positioning Faraday shields within the process chamber, but the weak plasma-drive-current coupling results in the formation of large eddy currents in the shields, which in turn produces substantial power dissipation. An alternative approach, such as described in WO 99/00823, entitled xe2x80x9cTOROIDAL LOW-FIELD REACTIVE GAS SOURCE,xe2x80x9d incorporated herein by reference, attempts to exploit a specific transformer arrangement in a toroidal RF plasma source. Semiconductor switching devices are used to drive the primary winding of a power transformer that couples electromagnetic energy to the plasma, thereby forming a secondary circuit of the transformer.
Toroidal plasma-source devices such as that described in WO 99/00823 have a number of limitations that it is desirable to overcome. For example, they are typically designed for only a specific load, thereby having limited operational flexibility. They are, moreover, restricted to operation at low RF frequencies (typically about 400 kHz), and require the use of a magnetic core, which contributes to efficiency losses. They also require an auxiliary starter to initiate plasma formation and require a flow of inert gas, such as Ar, to maintain the plasma. Such limitations are overcome with the present invention.
Embodiments of the invention are directed to an electrostatically shielded toroidal plasma source that does not use a magnetic core. Instead, the operation of the plasma source is achieved by direct inductive coupling between a current in a driving coil with the plasma current in the plasma chamber. The toroidal plasma source according to embodiments of the invention can be operated at high RF frequencies, i.e. greater than 400 kHz, with only water cooling. Plasma formation is achieved without the need for an auxiliary starter and without the need for including a flow of inert gas. The toroidal plasma source can accordingly be configured with a substrate processing system to achieve improved overall efficiency.
In a first embodiment, a metallic plasma source chamber defines an interior for plasma generation. The plasma source chamber includes at least one dielectric break. A drive inductor is configured such that the metallic plasma source chamber is positioned between loops of the drive inductor. An input coil is configured proximate the drive inductor to provide a mutual inductance between the input coil and the drive inductor. In one embodiment, the plasma source chamber is configured from two L-shaped portions assembled to form a rectangularly shaped enclosure. The dielectric break is defined by a gap between the two L-shaped portions. In one embodiment, the metallic plasma source chamber is grounded.
In another embodiment, the interior of the plasma source chamber is lined with a material that can be heated by the plasma, such as quartz. The liner acts to reduce losses due to oxygen recombination on surfaces, thereby improving the efficiency of substrate-processing operations.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and the attached figures.